Raman-active phonons in thina- andc-axis-oriented (YBa2Cu3O7)m(PrBa2Cu3O7)nsuperlattices

We report on a Raman study that compares the behavior of phonons in several a-axis- and c-axis-oriented superlattices of (${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ ${)}_{\mathit{m}}$(${\mathrm{PrBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ ${)}_{\mathit{n}}$. The number of individual layers within a superlattice period varied between 1≤m,n≤15. The dependence of the phonon frequencies on m and n shows a significant softening of the vibration associated with the apical oxygen atom [O(4)] in ${\mathrm{PrBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ with reductions in the Pr layer thickness (${\mathit{d}}_{\mathrm{Pr}}$). The modifications to the self-energy of this phonon for both superlattice orientations only occur when ${\mathit{d}}_{\mathrm{Pr}}$≲25 Å and therefore establishes the primary importance of the Pr layer thickness in affecting the observed softening. These phonon renormalizations are consistent with charge redistribution in the immediate vicinity (∼10 Å) of the interface arising from the unequal Fermi energies associated with the alternating layers. Our study also establishes that the significant differences in the dependence of the superconducting transition temperatures on layer thickness in a- and c-axis-oriented multilayers are not attributable to enhanced interdiffusion between Y and Pr sites or greater loss of oxygen in the a-axis structures. We also find no evidence of a Raman excitation proposed to be a combination ${\mathit{B}}_{1\mathit{u}\mathrm{-}}$ ${\mathit{B}}_{1\mathit{g}}$-type vibration recently reported in a c-axis oriented (${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ ${)}_2$(${\mathrm{PrBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ ${)}_1$ superlattice.